In infants, conventional ventilatory assistance often leads to pneumothorax since neonatal lungs are more subject to barotrauma than adult lungs. Because of relatively lower airway pressures in comparison to conventional mechanical ventilation, high frequency jet ventilation (HFJV) has been suggested as one method of assisting this patient population. Unfortunately, HFJV has been reported to lead to new traumatic effects including endothelial erosion due to the high velocity jet impinging on the tracheal endothelial surface, and gradual deterioration in overall gas exchange efficiency. Also, results obtained using HFJV in neonates have been difficult to compare due to the wide range of jet design and operation parameters employed by different clinical groups. Central to all of these problems is a lack of a clear understanding of the physical mechanisms of operation of HFJV in neonates. In order to obtain some of this understanding, research is proposed to investigate the hypothesis that jet nozzle design and location exert a major influence on the overall efficiency of gas exchange and tracheal wall shear rates in high frequency jet ventilation (HFJV) in neonates. We propose to test this hypothesis by using the experimental technique of liquid-bead flow visualization in physical scale models of the neonatal bronchial tree. Flow visualization will be achieved by taking conventional and high speed motion pictures of the oscillating liquid flow produced by the jet operating in the models. The liquid-bead flow in the physical models is made similar to the gas flow occurring during HFJV in neonatal lungs by use of the fluid mechanical principle of dynamic and geometrical similarity. The motion picture frames taken of the flow will be developed and analyzed using an image analyzer. By this analysis we will determine the tidal volume actually injected by the jet; the shearing force exerted by the jet on the tracheal wall; and the overall efficiency of the convective airway gas exchange process for a given set of parameters of jet design, operation and location. Pressures at various locations within the liquid-filled models will also be measured simultaneously and used to predict the corresponding gas pressures in the neonatal airways. The principle investigators have developed the technique of liquid-bead flow visualization in bronchial airway models and have applied it with considerable success to the case of normal respiration and high frequency-piston oscillation ventilation.